[app] make APP_START work with C++

Needs a __USED tag or the compiler will GC it apparently. Should do the
same thing for other section based trickery.

app/include/app.h

@@ -40,7 +40,7 @@ struct app_descriptor {
     size_t stack_size;
 };
 
-#define APP_START(appname) const struct app_descriptor _app_##appname __ALIGNED(sizeof(void *)) __SECTION("apps") = { .name = #appname,
+#define APP_START(appname) const struct app_descriptor _app_##appname __USED __ALIGNED(sizeof(void *)) __SECTION("apps") = { .name = #appname,
 
 #define APP_END };